Ultrasonic echoes from deliberately launched diagnostic sound waves into tissue are attenuated in proportion to the distance that the sound waves must travel to reach the reflector, plus the distance that the resulting echoes must travel back to reach the receiver. Since sound waves are attenuated as they pass through the human body, the deeper the penetration, the greater the attenuation.
Typically, the aperture of an ultrasonic imaging transducer opens wider as echoes are received from deeper depths. Aperture is defined as the lateral length (the longer dimension of the transducer surface) of the array of elements that are actively receiving.
Existing ultrasound imaging systems limit the ratio of the focus point depth to aperture length (i.e., the F number) to a value such as 2.0 or more. However, at shallow depths, some elements are too far from the focal point to achieve this F number and, hence, must be inactive. More elements turn on as the depth increases, causing the signal strength from the sum of all the active elements to increase. If the increase in signal strength were not compensated, then the image would appear to brighten with increasing depth, forcing the operator to compensate with time gain compensation (TGC). Since it is not desirable to have the operator correct for a system artifact, the correction must be done automatically. TGC is a method of increasing the receiver gain as echoes are received from deeper tissues or equivalently with time. Existing TGC's are analog, since the architecture of existing medical ultrasound systems is analog. However, ultrasound imaging systems are being developed which include digital architecture.
It would be desirable then to have a dynamic apodization correction technique for use with any ultrasound imaging system, including an ultrasound imaging system which incorporates digital architecture, which can maintain constant overall signal strength.